The hydraulic pressure created by the GMC brake master cylinder is the force that stops your truck from proceeding down the road. When the brake master cylinder malfunctions, the stopping of the truck can only be accomplished with the use of the parking brake; which is only intended for parking on hills or emergencies. All GMC brake master cylinders are located on the upper fire wall on the driver side under the hood of your truck. The top portion of this device has a cover. When the cover is removed the brake fluid reservoirs are exposed. In the bottom of the reservoirs is the brake master cylinder piston. It is the seals on this piston that can cause this braking component to fail on your GMC truck. A brake master cylinder that is failing will have the symptoms of a soft brake pedal that could reach the floor of the passenger compartment if pressure is continuously applied to it. In some instances the pumping of the brake pedal will produce enough pressure in the system to bring the truck to a stop, but not always. When the seals on this piston go out, the pressure being applied to the system will be released by the break in the seals. If this occurs on your GMC truck the replacement of this part is imperative for the safety of the occupants and others on the road near this vehicle. The failing brake master cylinder is the only component of the braking system that will have no exterior signs of malfunctioning by the formation of hydraulic fluid on the pavement. The leak is an interior one and must be properly diagnosed for the vehicle to be safe to operate your GMC truck.